The present invention relates to a waste rubber processing machine and method, and more particularly to a process and apparatus for grinding and reclaiming rubber or similar material.
Grinding or size reduction and crumb production of waste cured rubber, including scrap tires, is subject of numerous patents, articles and books.
There are a lot of plants in the world (ca 1200), which produce rubber crumb from the scrap tires. Most of them are dealing with the treatment of waste rubber by collecting, sorting and breaking it down to the required size as defined by the end user. Yet very few rubber industrialists deal with reclaiming and de-vulcanization.
However, at present the most acute challenge is what to do with this rubber crumb. According to published data, 51% of crumb is incinerated, 43% is added to asphalt and only 6% are rejuvenated as new raw rubber material.
Investigations performed at most Rubber Centers, such as Akron Rubber Development (USA), Kharagpur Rubber Institute (India), NIISP (Russia) have shown that main properties of rubber compound decrease linearly as content of ground rubber grows. It indicates obviously that granulated rubber behaves as filler in rubber compounds and cannot be used at high percentages. By data of Smith, upper level of mechanically ground crumb content cannot exceed 2.3% for radial tires and 3.2% for bias tires. It is caused by steep impair of main properties at major contents.
Various machines and technologies are used for production of cured rubber crumb.
One of the common known approaches is size reduction of rubber chips at breaking or grinding 2-roll mills. This technique is based on passing rubber chips through tight nip between rollers are widely used in the recycling industry. Technology based on multiple passing of raw rubber material mixed with special additives (modifier) through the narrow nip of 2-roll mills are suitable for devulcanization, too. The following patents: U.S. Pat. No. 3,269,468; U.S. Pat. No. 5,257,742; U.S. Pat. No. 2,211,518; U.S. Pat. No. 1,653,472; U.S. Pat. No. 4,747,550 and U.S. Pat. No. 1,758,010 describe different grinding mills. Low productivity caused by very short time of shear stress in the nip (0.04-0.05 sec) restricts feasibility of this technology.
Another technique is based on knife cutting as disclosed by U.S. Pat. No. 5,695,131; U.S. Pat. No. 5,299,744 and U.S. Pat. No. 7,021,576. Multi-knife shredder or rotor knife mill are widely used in rubber recycling. Though this technique provides relatively high productivity, it yields only rough crumb and, in addition to that, cut crumb is contaminated by metal impurities.
Technique involving freezing of rubber (cryogenic methods) prior to or during processing in mills are known in the art. U.S. Pat. No. 4,813,614 and GB patent 1559152 disclose cryogenic grinding between two wheels provided by a set of crushing pins. U.S. Pat. No. 4,383,650 discloses grinding of frozen material between two toothed wheels. It is also known cryogenic milling in vibration mill or a rotary blade mill, where the material is maintained cooled below −60 .degree C. by supplying a coolant, particularly liquid nitrogen or carbonic acid. Cryogenic grinding provides rather fine particles with minimum impurities. However, extremely high cost of liquid nitrogen makes cryogenic methods uneconomical. Besides, cryogenic technology does not allow devulcanization by chemical agents, because these reactions do not occur at low temperatures.
In the last decades use of extruders for rubber grinding has been developed widely, as disclosed in U.S. Pat. No. 6,590,042; U.S. Pat. No. 6,576,680 and U.S. Pat. No. 6,335,377. Extruder could operate within a wide range of temperatures (up to 300° C.) and provides sufficient productivity. The great skill is required for adjusting the mill to achieve optimum results by this technology, because the material is intensely heated during processing in the mill and the extruder. Thus, a great amount of heat energy must be dissipated by water-cooling. Disadvantages of this technique are: lack of possibility for ultra-fine grinding because clearance between screw and walls cannot be performed tightly enough; unfeasibility for effective cooling of solid bulk of processed material; extreme energy consumption.
Attrition or disc mills for rubber grinding also are known in the art. U.S. Pat. No. 4,535,941 for example, discloses grinding at horizontal mill, similar to those used in pigments industry. Later U.S. patent of the same inventor, U.S. Pat. No. 4,625,922 discloses improved technique, of grinding at elevated temperatures. This technique suffers from unfeasibility of continuous process. In order to overcome this drawback, it was proposed to perform rubber grinding in flow of liquid slurry (water, oil, etc.) as described in U.S. Pat. No. 5,238,194, U.S. Pat. No. 5,411,215 and U.S. Patent Application U.S. 20020086911. Sufficient continuity could be achieved; however necessity in consequent drying of the produced crumb makes such a process impractical.
Construction of disc mill for grinding rubber between grinding wheel and shroud is also disclosed in U.S. Pat. No. 6,976,646. Setting of uniform clearance between wheel and shroud along entire perimeter of working zone at this design is problematic.
Therefore, a need still exists for an improved technique of producing commercially reusable rubber material by providing a cost-effective, continuousgenerally ambient temperature process.